"While there is some evidence to support the keto approach in relation to short term outcomes, there still really isn't enough solid evidence to know how this dietary approach impacts our health in the longer term," nutritionist Kristen Beck told news.com.au.

"At a practical level, the keto diet is very hard to implement and stick to, and can actually have detrimental outcomes if not followed correctly."

The diet, which has Guy Sebastian and Kim Kardashian as fans, puts the dieter into "ketosis," when the body breaks down both ingested and stored body fat into ketones, which it uses as energy. People on such diets often deal with fatigue and light-headedness as they adjust to a lack of carbohydrates.

Though the experts on the US News and World Report panel that created the list said eating that way isn't harmful short-term, they ranked the diets poorly on long-term weight loss success, ease of use and overall impact on health.

For the relatively new keto diet, the experts were especially concerned about extremely high fat content - about 70 per cent of daily calorie intake - as well as unusually low carbohydrate levels: only 15 to 20 net carbs a day. The 2015-20 dietary guidelines for Australians suggest that 45 per cent to 65 per cent of daily calories come from carbs but less than 10 per cent from saturated fat.

"When you are on the keto diet, you drastically cut your carbs to only 20 per day. That's less than one apple!" said nutritionist Lisa Drayer, told CNN.

"The keto diet is just not sustainable over the long term. It doesn't teach you how to acquire healthy eating habits. It's good for a quick fix, but most people I know can hardly give up pasta and bread, let alone beans and fruit."

The expert panel was particularly concerned for people with liver or kidney conditions, "who should avoid it altogether," the report said, adding that there was not enough evidence to know whether the diet would help those with heart issues or diabetes. Because of the recommended "cycling" nature of the diet, taking breaks and then starting it again, experts also warn that hormones could fluctuate.

Guy Sebastian did the Keto diet in 2017.

First place for best diet is a tie

For the first time, the well-researched Mediterranean diet moved into first place, tied with the DASH diet. DASH, which stands for Dietary Approaches to Stop Hypertension, has held the top spot for eight consecutive years. Both diets also tied for healthiest in the rankings.

"I'm very glad to see the Mediterranean and DASH diets come out on top," Ms Beck said.

"Best things about both of these diets is that their focus is on wholesome, fresh produce, within an overall healthy lifestyle.

"In particular both of these diets focus on fresh vegetables and fruits as their basis - which should be the basis on any healthy diet."

Drayer said the best part about the Mediterranean diet is the inclusion of red wine as part of the eating plan.

The DASH diet is often recommended to lower blood pressure. Its premise is simple: Eat more veggies, fruits and low-fat dairy foods while cutting way back on any food high in saturated fat and limit your intake of salt.

The Mediterranean encourages red wine. Yesss.

The meal plan includes three wholegrain products each day, four to six servings of vegetables, four to six servings of fruit, two to four servings of dairy products and several servings each of lean meats and nuts/seeds/legumes.

Studies have shown that following this diet can reduce blood pressure in a matter of weeks.

The Mediterranean diet also ranked first on the US News and World Report list for easiest diet to follow, best plant-based diet and best diet for diabetes. It came in second for best heart-healthy diet, just behind DASH.

Meals from the sunny Mediterranean have been linked to stronger bones, a healthier heart, a lower risk of dementia and breast cancer, and longer life, along with a reduced risk for diabetes and high blood pressure.

The diet emphasises simple, plant-based cooking, with the majority of each meal focused on fruits and vegetables, whole grains, beans and seeds, with a few nuts and a heavy emphasis on extra virgin olive oil. Say goodbye to refined sugar or flour. Fats other than olive oil, such as butter, are consumed rarely, if at all.

Meat can make a rare appearance, but usually only to flavour a dish. Meals may include eggs, dairy and poultry, but in much smaller portions than in the traditional Western diet. However, fish are a staple, and an optional glass of wine with dinner is on the menu.

The flexitarian diet came in third on the list of best overall diets for its emphasis on whole grains, fruits, veggies and plant-based proteins. It's basically a vegetarian diet that allows the occasional piece of meat or fish, thus making it "flexible."

The flexitarian diet, which involves the occasional consumption of meat, came in as the third best diet.

Best weight-loss diets

Weight Watchers topped the list of best weight-loss and best commercial diet plans, and it tied with HMR for best fast weight-loss diet. HMR, the Health Management Resources program, involves purchasing meal replacements, such as shakes, nutrition bars and multigrain cereals, and adding vegetables and fruits to round out the meals.

Since 2015, Weight Watchers has been focused on its Beyond the Scale program. It is designed to help people move more and focus on overall wellbeing, including more "me" time, which includes mindfulness and self-compassion.

The US News and World Report expert panel - made up of 25 experts from major medical centres across the country - gave Weight Watchers top points for being healthy, also stressing the importance of support for dieters.

Pointing to a 2006 British Medical Journal study, the panel said that "programs like Weight Watchers that offer emotional support and group meetings lead to higher compliance than a do-it-yourself dieting."

The best diets of 2018 have been released.

Drayer agreed: "I've always been a fan of Weight Watchers because it teaches you how to incorporate all types of foods into your diet, and it also offers a support component, which can be extremely beneficial."

But whatever diet you choose, she recommends choosing the one that fits your lifestyle and that you can stick to over the long term.

"I always recommend meeting with a registered dietitian, who can take into account your health history, lifestyle habits, likes and dislikes, and develop an eating plan that is specific to your needs," she added. "I also recommend checking with your doctor first before starting any new diet plan."